The invention relates to the field of semiconductor device protection circuits, and more particularly to the field of such protection circuits which prevent damage to the semiconductor device by limiting the maximum amount of power dissipatioon and/or current permitted in the semiconductor device.
In prior protection circuits, a signal is developed related to the amount of power dissipation or current occurring in a semiconductor device. This signal is then compared to a fixed limit signal, and the device is turned off, or excitation to the device is reduced, so as to reduce power dissipation or current in the device when the limit has been exceeded. While such circuits do prevent excessive power dissipation in the semiconductor device, sometimes, such as for low device temperatures, the semiconductor device could actually dissipate additional power without any damage to the damage. However, since the protection device must guard against a worst case situation, a conservative fixed power dissipation/current limit is implemented for the semiconductor device. Thus many times the normal operation of the semiconductor device at low temperatures may be unnecessarily interrupted by reducing the power dissipation/current of the device even though this reduction is not necessary to protect the device.
Some prior protection circuits have developed a signal indicative of the temperature of a semiconductor device. These circuits then reduce power dissipation of (current through) the semiconductor device just in response to the sensed temperature of the device exceeding some maximum predetermined fixed temperature limit value. The sensed temperature signal produced in these prior circuits is provided by a temperature sensing element separate from the semiconductor device and therefore may not accurately be representative of the semiconductor device actual temperature. Thus, a substantial safety margin must be designed into such protection circuits to ensure that excessive power dissipation of the semiconductor device does not occur because of variable poor thermal coupling between the semiconductor device and the additional temperature sensing element. Also, such protection circuits really do not provide a power dissipation limit, but actually just provide a maximum temperature limit since they just monitor device temperature and not power dissipation. In addition, such circuits typically require extensive additional circuitry to provide any additional short circuit overcurrent limitation for the device.
Some prior protection circuits develop a signal related to the current passed through a semiconductor device. This signal is developed either by monitoring the voltage across an added current sense resistor connected in series with the device or by monitoring the voltage across the device. In either case, the current sense signal is just compared to a fixed reference threshold. Thus, as noted above, these circuits may unnecessarily interrupt normal operation of the device at low temperatures. Also, if these prior circuits reduce radio frequency interference by slowing down the rate of device switching, this will result in slowing down the circuit response to detected overcurrent conditions, and this isn't desirable.